Polymer latices, also termed polymer emulsions, are widely used in industrial applications, including binders for paints, printing inks, non-woven fabrics and the like, paper coatings and the like. These latices may be prepared in continuous or batch processes by polymerizing monomers, usually ethylenically unsaturated compounds, in the presence of water, surfactants and other adjuvants that affect the manufacturing process or the properties of the latices.
Economics may dictate that the same kettles, piping and other equipment be used to produce different latices, so the equipment must be cleaned between batches. Even where a single latex is produced on a continuous basis, the equipment must still be cleaned periodically.
Cleaning usually comprises washing the equipment with water; this creates large volumes of dilute aqueous latex known as whitewater. Whitewater thus created normally has a solids concentration of about 5% by weight or less, although it may be higher. This solids concentration representing emulsion-sized particles of the original polymer product. In addition to these submicroscopic polymeric particles of the latex, whitewater may also contain alcohols or other organic liquids, surfactants and the like. As produced, the solids concentration of the whitewater emulsion is far below the typical 40% or greater found in the original polymer latex, but it represents enough suspended organic matter to cause a serious waste-disposal problem.
Typical whitewaters may contain emulsion-sized particles of polymers such as styrenics, acrylics such as polymers of esters of acrylic or methacrylic acids, acrylonitrile, vinyl polymers such as poly(vinyl chloride), and complex copolymers of two or more such materials, with crosslinders, graftlinkers and the like, such as butadiene, divinylbenzene, ethylene glycol dimethacrylate, allyl methacrylate and the like.
In typical manufacturing operations, the whitewater generated by batches of different polymer types are combined, and the entire mixture is treated as a single waste stream. To reduce the volume of waste, the whitewater is frequently concentrated before disposal, typically by chemical coagulation, coarse filtration, and in some cases ultrafiltration. The concentrated or coagulated waste, which is a mixture of whatever polymers the equipment happened to be making, plus cleaning agents and miscellaneous contaminants, is then typically buried in land-fill, or used as filler in asphalt or as a dust-control agent on roadways.
Semipermeable membrane filtration, and in particular ultrafiltration, has been employed to concentrate polymer emulsions or latices. In the ultrafiltration process a latex is pumped into the inlet end of a hollow membrane fiber, or cartridge comprising several of these fiber in parallel; the walls of these tubes are "semipermeable", that is, they allow materials of low molecular weight to pass through, but are impermeable to higher-molecular-weight materials such as polymer. The pumped latex flows through the hollow lumen of the membrane fiber parallel to its walls; this flow is known as "cross flow". As the latex transits the lumen of the membrane fiber, water, salts, surfactants and other low-molecular-weight materials pass from the latex through the walls of the membrane. The flow rate through the membrane wall per unit of membrane surface area is the membrane "flux", and the liquid which has passed through the membrane wall is called the "permeate". The polymer and other high-molecular-weight materials which do not pass through the membrane wall appear in the "retentate", which emerges along with some of the water from the exit end of the membrane fiber or cartridge under the pumping pressure, and is recycled through the fiber or cartridge until the desired concentration is reached. Transmembrane pressures, that is, pressures across the membrane wall, are typically from about 70 to about 1400 kiloPascals (kPa), more typically from about 140 to about 700 kPa. Hydrodynamic pressures, that is, pressures across the length of the membrane fiber or cartridge, depend upon the viscosity of the latex at the operating temperature, and are typically in the same range as for the transmembrane pressures. Temperatures for the ultrafiltration process are typically within the range of about 5.degree. C. to about 70.degree. C., and more typically about 10.degree. C. to about 40.degree. C.
The above description of ultrafiltration is based upon the ultrafiltration membrane being configured as a hollow fiber. Ultrafiltration membranes may also be configured as larger tubes or as sheets, which may be used singularly or in pairs with the active membranes facing one another and the liquid to be treated being passed between them; such sheets may be used flat or wound into spiral tubes. Other configurations are known to those skilled in the art.
The latex is sheared as it is pumped through the ultrafiltration system. Sources of shear include the pump or other device used to propel the whitewater through the system, and the ultrafiltration cartridge itself; shear occurs as the whitewater is forced under pressure into the relatively small inlet port or ports into the cartridge, and as the walls of the membrane resist the flow of the whitewater. This mechanical shearing contributes to destabilizing the latex and forming a coagulum, or aggregate of polymeric latex particles, which fouls the membrane surface and pores, reducing flux rate through the membrane. The ultrafiltration process also removes some of the water from the aqueous phase and removes surfactant from the polymer latex, which also helps destabilize the latex. Such destabilized latices do not retain their original performance properties, and must be regarded as low grade product or waste.
As a result, previous attempts at concentrating whitewater by ultrafiltration produced mixed success, because many latices proved to be unsuited for the process. The flux, which was initially satisfactory, deteriorated rapidly because of the fouling described above. The membranes required frequent cleaning, for instance by washing them with surfactants or solvents, as described in U.S. Pat. No. 3,956,114, to remove the fouling and at least partially restore the flux rate. This frequent cleaning not only removed the system from service, reducing the overall throughput of the system, but also was only partially effective, so the overall life of the membrane filter was often unsatisfactorily short.
In U.S. Pat. No. 4,160,726, the problem of coagulum formation, as it relates to fouling, was addressed by adding surfactant to the whitewater latex prior to or during the concentration process, in an attempt to stabilize the latex. While partially successful, this approach did not necessarily work for all whitewater latices, and did not address the change in properties of the retained latex.
An object of the present invention is to provide a process by which the polymer latices recovered from whitewater may be recycled into high-value product instead of being treated as low-value waste and by-product. Another object of the present invention is to provide an apparatus to recover such high-value product, and yet another object is to provide the high-value, polymeric product so recovered. Other objects of the invention will be apparent from the specification and claims which follow.